Measurement and Control of Dissolved Oxygen below 100 ppb

Oxygen is important in almost any fermentation. Because of its low solubility it often will be limiting in industrial processes. In spite of its great importance, there is rather little quantitative information on the influence of oxygen on microbial metabolism.l4 This is due to the fact that very often dissolved oxygen (DO) will influence kinetics a t rather lower levels (-100 ppb) only, which under fermentation conditions is not easily measured by any of the existing methods. All known existing methods involve membranes to separate the actual detector (gas sensor for tubing method, electrochemical detector, mass spectrometer) from the harsh and complex environments.' Wimpenny' suggested the use of the redox potential for characterization of processes with rather low DO because of its higher sensitivity to D O in this region. However, redox potential has the disadvantage that it is influenced by a series of redox pairs, and thus is very sensitive to most medium changes.' The electrode must have a proper electrical design and installation, and transport processes a t the membrane interface must be well understood and defined.' It has been reported that bubbles may cause artifacts when directly contacting the membrane."," For kinetic studies it would be very desirable not only to measure but also to control DO at predetermined levels. A number of control methods have been described in the literature4.'*-":